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Date: Thu, 13 May 2021 11:58:38 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Alexey Dobriyan <adobriyan@...il.com>, mingo@...hat.com,
Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
x86@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/4] sched: make nr_running() return 32-bit
* Thomas Gleixner <tglx@...utronix.de> wrote:
> Alexey,
>
> On Thu, Apr 22 2021 at 23:02, Alexey Dobriyan wrote:
> > Creating 2**32 tasks is impossible due to futex pid limits and wasteful
> > anyway. Nobody has done it.
> >
>
> this whole pile lacks useful numbers. What's the actual benefit of that
> churn?
I applied the 4 patch series from Alexey Dobriyan because they offer four
distinct technical advantages to the scheduler code:
- Shorter instructions generated in nr_running(), nr_iowait(),
nr_iowait_cpu() due to losing the REX prefix.
- Shorter instructions generated by users of these 3 functions as well.
- Tighter data packing and structure size reduction in 'struct rt_rq',
'struct dl_rq' and 'struct rq', due to 8-byte 'long' fields shrinking to
4-byte 'int' based fields.
- Together these 4 patches clean up all derivative uses of the
rq::nr_running base type, which is already 'unsigned int' and that's
pretty fundamental given our PID ABI limits. Having type mismatch where
we use 64-bit data types for certain APIs while 32-bit data types for
others is inconsistent crap I wouldn't accept if it was submitted as new
code.
As to the numbers:
The data structure size improvements are IMO obvious, and they are also
measurable, here's the before/after Pahole comparison of 'struct rt_rq':
--- pahole.struct.rt_rq.before 2021-05-13 11:40:53.207077908 +0200
+++ pahole.struct.rt_rq.after 2021-05-13 11:41:42.257385897 +0200
@@ -7,22 +7,22 @@ struct rt_rq {
int curr; /* 1624 4 */
int next; /* 1628 4 */
} highest_prio; /* 1624 8 */
- long unsigned int rt_nr_migratory; /* 1632 8 */
- long unsigned int rt_nr_total; /* 1640 8 */
- int overloaded; /* 1648 4 */
+ unsigned int rt_nr_migratory; /* 1632 4 */
+ unsigned int rt_nr_total; /* 1636 4 */
+ int overloaded; /* 1640 4 */
/* XXX 4 bytes hole, try to pack */
- struct plist_head pushable_tasks; /* 1656 16 */
- /* --- cacheline 26 boundary (1664 bytes) was 8 bytes ago --- */
- int rt_queued; /* 1672 4 */
- int rt_throttled; /* 1676 4 */
- u64 rt_time; /* 1680 8 */
- u64 rt_runtime; /* 1688 8 */
- raw_spinlock_t rt_runtime_lock; /* 1696 4 */
+ struct plist_head pushable_tasks; /* 1648 16 */
+ /* --- cacheline 26 boundary (1664 bytes) --- */
+ int rt_queued; /* 1664 4 */
+ int rt_throttled; /* 1668 4 */
+ u64 rt_time; /* 1672 8 */
+ u64 rt_runtime; /* 1680 8 */
+ raw_spinlock_t rt_runtime_lock; /* 1688 4 */
- /* size: 1704, cachelines: 27, members: 13 */
- /* sum members: 1696, holes: 1, sum holes: 4 */
+ /* size: 1696, cachelines: 27, members: 13 */
+ /* sum members: 1688, holes: 1, sum holes: 4 */
/* padding: 4 */
- /* last cacheline: 40 bytes */
+ /* last cacheline: 32 bytes */
'struct rt_rq' got shrunk from 1704 bytes to 1696 bytes, an 8 byte
reduction.
'struct dl_rq' shrunk by 8 bytes:
- /* size: 104, cachelines: 2, members: 10 */
- /* sum members: 100, holes: 1, sum holes: 4 */
- /* last cacheline: 40 bytes */
+ /* size: 96, cachelines: 2, members: 10 */
+ /* sum members: 92, holes: 1, sum holes: 4 */
+ /* last cacheline: 32 bytes */
'struct rq', which embedds both rt_rq and dl_rq, got 20 bytes smaller:
- /* sum members: 2967, holes: 10, sum holes: 137 */
+ /* sum members: 2947, holes: 11, sum holes: 157 */
Side note: there's now 20 bytes more new padding holes which could possibly
be coalesced a bit more by reordering members sensibly - resulting in even
more data footprint savings. (But those should be separate changes and only
for fields we truly care about from a performance POV.)
As to code generation improvements:
> Just with the default config for one of my reference machines:
>
> text data bss dec hex filename
> 16679864 6627950 1671296 24979110 17d26a6 ../build/vmlinux-before
> 16679894 6627950 1671296 24979140 17d26c4 ../build/vmlinux-after
> ------------------------------------------------------------------------
> +30
Using '/usr/bin/size' to compare before/after generated code is the wrong
way to measure code generation improvements for smaller changes due to
default alignment creating a reserve of 'padding' bytes at the end of most
functions. You have to look at the low level generated assembly directly.
For example, the nr_iowait_cpu() commit, with before/after generated code
of the callers of the function:
9745516841a5: ("sched: Make nr_iowait() return 32-bit value")
ffffffffxxxxxxxx: e8 49 8e fb ff call ffffffffxxxxxxxx <nr_iowait_cpu>
- ffffffffxxxxxxxx: 48 85 c0 test %rax,%rax
ffffffffxxxxxxxx: e8 64 8e fb ff call ffffffffxxxxxxxx <nr_iowait_cpu>
+ ffffffffxxxxxxxx: 85 c0 test %eax,%eax
Note how the 'test %rax,%rax' lost the 0x48 64-bit REX prefix and the
generated code got one byte shorter from "48 85 c0" to "85 c0".
( Note, my asm generation scripts filter out some of the noise to make it
easier to diff generated asm, hence the ffffffffxxxxxxxx placeholder. )
The nr_iowait() function itself got shorter by two bytes as well, due to:
Before:
ffffffffxxxxxxxx <nr_iowait>:
ffffffffxxxxxxxx: 41 54 push %r12
ffffffffxxxxxxxx: 48 c7 c7 ff ff ff ff mov $0xffffffffxxxxxxxx,%rdi
ffffffffxxxxxxxx: 45 31 e4 xor %r12d,%r12d
ffffffffxxxxxxxx: 55 push %rbp
ffffffffxxxxxxxx: 8b 2d 01 ea 5d 01 mov 0x15dea01(%rip),%ebp # ffffffffxxxxxxxx <nr_cpu_ids>
ffffffffxxxxxxxx: 53 push %rbx
ffffffffxxxxxxxx: 48 c7 c3 c0 95 02 00 mov $0x295c0,%rbx
ffffffffxxxxxxxx: eb 17 jmp ffffffffxxxxxxxx <nr_iowait+0x34>
ffffffffxxxxxxxx: 48 98 cltq
ffffffffxxxxxxxx: 48 8b 14 c5 a0 c6 2e mov -0x7dd13960(,%rax,8),%rdx
ffffffffxxxxxxxx: 82
ffffffffxxxxxxxx: 48 01 da add %rbx,%rdx
ffffffffxxxxxxxx: 48 63 82 98 09 00 00 movslq 0x998(%rdx),%rax
ffffffffxxxxxxxx: 49 01 c4 add %rax,%r12
ffffffffxxxxxxxx: 48 c7 c6 18 72 67 82 mov $0xffffffffxxxxxxxx,%rsi
ffffffffxxxxxxxx: e8 80 46 39 00 call ffffffffxxxxxxxx <cpumask_next>
ffffffffxxxxxxxx: 89 c7 mov %eax,%edi
ffffffffxxxxxxxx: 39 e8 cmp %ebp,%eax
ffffffffxxxxxxxx: 72 d7 jb ffffffffxxxxxxxx <nr_iowait+0x1d>
ffffffffxxxxxxxx: 4c 89 e0 mov %r12,%rax
ffffffffxxxxxxxx: 5b pop %rbx
ffffffffxxxxxxxx: 5d pop %rbp
ffffffffxxxxxxxx: 41 5c pop %r12
ffffffffxxxxxxxx: c3 ret
After:
ffffffffxxxxxxxx <nr_iowait>:
ffffffffxxxxxxxx: 41 54 push %r12
ffffffffxxxxxxxx: bf ff ff ff ff mov $0xffffffff,%edi
ffffffffxxxxxxxx: 45 31 e4 xor %r12d,%r12d
ffffffffxxxxxxxx: 55 push %rbp
ffffffffxxxxxxxx: 8b 2d 03 ea 5d 01 mov 0x15dea03(%rip),%ebp # ffffffffxxxxxxxx <nr_cpu_ids>
ffffffffxxxxxxxx: 53 push %rbx
ffffffffxxxxxxxx: 48 c7 c3 c0 95 02 00 mov $0x295c0,%rbx
ffffffffxxxxxxxx: eb 17 jmp ffffffffxxxxxxxx <nr_iowait+0x32>
ffffffffxxxxxxxx: 48 63 c7 movslq %edi,%rax
ffffffffxxxxxxxx: 48 8b 14 c5 a0 c6 2e mov -0x7dd13960(,%rax,8),%rdx
ffffffffxxxxxxxx: 82
ffffffffxxxxxxxx: 48 01 da add %rbx,%rdx
ffffffffxxxxxxxx: 8b 82 98 09 00 00 mov 0x998(%rdx),%eax
ffffffffxxxxxxxx: 41 01 c4 add %eax,%r12d
ffffffffxxxxxxxx: 48 c7 c6 18 72 67 82 mov $0xffffffffxxxxxxxx,%rsi
ffffffffxxxxxxxx: e8 a2 46 39 00 call ffffffffxxxxxxxx <cpumask_next>
ffffffffxxxxxxxx: 89 c7 mov %eax,%edi
ffffffffxxxxxxxx: 39 c5 cmp %eax,%ebp
ffffffffxxxxxxxx: 77 d7 ja ffffffffxxxxxxxx <nr_iowait+0x1b>
ffffffffxxxxxxxx: 44 89 e0 mov %r12d,%eax
ffffffffxxxxxxxx: 5b pop %rbx
ffffffffxxxxxxxx: 5d pop %rbp
ffffffffxxxxxxxx: 41 5c pop %r12
ffffffffxxxxxxxx: c3 ret
The size of nr_iowait() shrunk from 78 bytes to 76 bytes.
Or the other commit:
01aee8fd7fb2: ("sched: Make nr_running() return 32-bit value")
ffffffffxxxxxxxx: e8 d9 24 e8 ff call ffffffffxxxxxxxx <nr_running>
ffffffffxxxxxxxx: 4c 8b 05 cc 92 70 01 mov 0x17092cc(%rip),%r8 # ffffffffxxxxxxxx <total_forks>
ffffffffxxxxxxxx: 4c 8b 64 24 50 mov 0x50(%rsp),%r12
- ffffffffxxxxxxxx: 48 89 44 24 08 mov %rax,0x8(%rsp)
ffffffffxxxxxxxx: 4c 89 04 24 mov %r8,(%rsp)
ffffffffxxxxxxxx: e8 f9 24 e8 ff call ffffffffxxxxxxxx <nr_running>
ffffffffxxxxxxxx: 4c 8b 05 ed 92 70 01 mov 0x17092ed(%rip),%r8 # ffffffffxxxxxxxx <total_forks>
ffffffffxxxxxxxx: 4c 8b 64 24 50 mov 0x50(%rsp),%r12
+ ffffffffxxxxxxxx: 89 44 24 08 mov %eax,0x8(%rsp)
ffffffffxxxxxxxx: 4c 89 04 24 mov %r8,(%rsp)
Note how "mov %rax,0x8(%rsp)" got shortened by one byte to "mov %eax,0x8(%rsp)":
- ffffffffxxxxxxxx: 48 89 44 24 08 mov %rax,0x8(%rsp)
+ ffffffffxxxxxxxx: 89 44 24 08 mov %eax,0x8(%rsp)
The nr_running() function itself got shorter by 2 bytes, due to shorter
instruction sequences.
The third commit improved code generation too:
8fc2858e572c: ("sched: Make nr_iowait_cpu() return 32-bit value")
ffffffffxxxxxxxx <nr_iowait_cpu>:
ffffffffxxxxxxxx: 48 63 ff movslq %edi,%rdi
ffffffffxxxxxxxx: 48 c7 c0 c0 95 02 00 mov $0x295c0,%rax
ffffffffxxxxxxxx: 48 03 04 fd a0 46 0f add -0x7df0b960(,%rdi,8),%rax
ffffffffxxxxxxxx: 82
- ffffffffxxxxxxxx: 48 63 80 98 09 00 00 movslq 0x998(%rax),%rax
ffffffffxxxxxxxx: c3 ret
ffffffffxxxxxxxx <nr_iowait_cpu>:
ffffffffxxxxxxxx: 48 63 ff movslq %edi,%rdi
ffffffffxxxxxxxx: 48 c7 c0 c0 95 02 00 mov $0x295c0,%rax
ffffffffxxxxxxxx: 48 03 04 fd a0 46 0f add -0x7df0b960(,%rdi,8),%rax
ffffffffxxxxxxxx: 82
- ffffffffxxxxxxxx: 8b 80 98 09 00 00 mov 0x998(%rax),%eax
ffffffffxxxxxxxx: c3 ret
Note how the 'movslq 0x998(%rax),%rax' lost the REX prefix and got one byte
shorter. Call sites got shorter too:
ffffffffxxxxxxxx: e8 e8 73 fa ff call ffffffffxxxxxxxx <nr_iowait_cpu>
- ffffffffxxxxxxxx: 48 85 c0 test %rax,%rax
ffffffffxxxxxxxx: e8 c8 73 fa ff call ffffffffxxxxxxxx <nr_iowait_cpu>
- ffffffffxxxxxxxx: 85 c0 test %eax,%eax
You often won't see these effects in the 'size vmlinux' output, because
function alignment padding reserves usually hide 1-2 byte size improvements
in generated code.
> I'm truly impressed by the massive savings of this change and I'm even
> more impressed by the justification:
>
> > Bring nr_running() into 32-bit world to save on REX prefixes.
>
> Aside of the obvious useless churn, REX prefixes are universaly true for
> all architectures, right? There is a world outside x86 ...
Even architectures that have the same instruction length for 32-bit and
64-bit data types benefit:
- smaller data structures benefit all 64-bit architectures
- the cleaner, more coherent nr_running type definitions are now more
consistently 'int' based, not the previous nonsensical 'int/long' mix
that also made the generated code larger on x86.
More importantly, the maintenance benchmark in these cases is not whether a
change actively helps every architecture we care about - but whether the
change is a *disadvantage* for them - and it isn't here.
Changes that primarily benefit one common architecture, while not others,
are still eligible for upstream merging if they otherwise meet the quality
threshold and don't hurt the other architectures.
TL;DR:
This benefits from this series are small, but are far from 'useless churn',
unless we want to arbitrarily cut off technically valid contributions that
improve generated code, data structure size and code readability, submitted
by a long-time contributor who has contributed over 1,300 patches to the
kernel already, just because we don't think these add up a significant
enough benefit?
No doubt the quality barrier must be and is higher for smaller changes -
but this series IMO passed that barrier.
Anyway, I've Cc:-ed Linus and Greg, if you are advocating for some sort of
cut-off threshold for small but measurable improvements from long-time
contributors, it should probably be clearly specified & documented in
Documentation/SubmittingPatches ...
Thanks,
Ingo
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